Metallocene catalysts, those organometallic compounds comprising a Group 4, 5 or 6 metal bound to at least one cyclopentadienyl or ligand isolobal to cyclopentadienyl, are well known olefin polymerization catalysts. While there are many advantages to using metallocenes in olefin polymerizations to produce polyethylenes, there are some drawbacks to using metallocenes. For some time it has been recognized that metallocene catalysts, in particular, supported metallocene catalysts, have poor flowability and tend to adhere to surfaces or to itself. Adding other reagents to the catalyst composition such as commonly known antifouling agents can compound this issue. This causes practical problems in storing, transporting, and then injecting the dry catalyst into a polymerization reactor. There have been some attempts in the art to solve such problems.
In U.S. Pat. No. 5,795,838, flowability problems associated with supported metallocene halides—those metallocenes having at least one halide leaving group bound to the metal center—was addressed by using a catalyst having certain levels of alkyl groups associated with the benzene-insoluble alkylalumoxanes used to form the supported catalyst compositions, and further, to prepolymerized the catalyst prior to using as a dry catalyst. While this was reported to improve flowability, it entails many steps and thus is not advantageous.
In order to address reactor fouling problems caused by high activity metallocene halides, other reagents are often added to the catalyst, either separately or as part of the supported catalyst composition, such as in U.S. Pat. Nos. 6,300,436 and 5,283,278. Such additives, however, often compound the problem of flowability of the catalyst. This problem in turn was addressed in U.S. Pat. No. 6,593,267 though heating the catalyst composition before or while combining with the additive. This solution, however, has still proven somewhat inadequate, as it adds to the cost and complexity of making the catalyst.
The inventors have surprisingly found that the synergistic use of metallocene-alkyls in conjunction with a support material having been calcined at a relatively high temperature (above 600° C.) solves these and other problems. While metallocene-alkyls—those metallocenes having at least one alkyl group bound (often referred to as the “leaving group”) to the metal center—are known, such as in E. Giannetti et al., “Homogeneous Ziegler-Natta Catalysts. II. Ethylene Polymerization by IVB Transition Metal Complexes/Methyl Alkylalumoxane Catalyst Systems”, in 23 J. POLY. SCI.: POLY. CHEM. ED. 2117-2133 (1985), and in U.S. Pat. No. 5,241,025, what is surprising is that such compounds can be used in conjunction with antifoulant agents to both improve reactor operability and catalyst flowability, as well as overall polymerization activity. The latter is particularly surprising given that it is known that zirconocene-alkyls are not as active as zirconocene-halides under the same conditions and using methalumoxane, as shown by N. V. Semikolenova et al., “On The Interaction of Supported Zirconocene Catalysts With Alkylaluminum Co-Catalysts”, in 198 MACROMOL. CHEM. PHYS. 2889-2897 (1997). Nonetheless, the inventors have found such advantages here.